Magnetic lens assemblies for corpuscular ray devices which operate under vacuum

ABSTRACT

A corpuscular ray device having a magnetic lens assembly. Although the invention is particularly applicable to objective lens assemblies of electron microscopes which operate under vacuum, it is also applicable to lens assemblies for ion microscopes, electron-diffraction devices, or other corpuscular ray apparatus wherein the rays are situated in tubular enclosures. The magnetic lens assembly includes a pair of shielding cylinders which are coaxial and spaced from each other, which are made of a superconductive material, and which have their common axis coinciding with a lens axis of the lens assembly, these shielding cylinders being surrounded by a lens winding which may also be made of a superconductive material and which serves to generate the magnetic field which is concentrated by the shielding cylinders in the region of the corpuscular ray. These shielding cylinders have directed toward each other a pair of end faces which are spaced from each other and define between themselves a lens gap which is devoid of any shielding components made of superconductive material. The aperture error constant of the lens assembly depends upon the maximum value of the field intensity in the lens gap and upon the field gradient along the lens axis in the lens gap. The magnitude of the lens gap is chosen in such a way that with a predetermined value of field intensity outwardly beyond the lens gap and the shielding cylinders the maximum value of the field intensity in the lens gap and the field gradient in the lens gap along the lens axis provide for the lens assembly an aperture error constant which is less than a predetermined value of aperture error constant.

United States Patent 1 Zerbst et al.

[ MAGNETIC LENS ASSEMBLIES FOR CORPUSCULAR RAY DEVICES WHICH OPERATEUNDER VACUUM [75] Inventors: Helmut Zerbst; Reinhard Weyl;

[solde Dietrich, all of Munich, Germany [73] Assignee: SiemensAktiengesellschaft,

Berlin and Munich. Germany [22] Filed: June 26, 1967 [2|] Appl. No.1648,623

[30] Foreign Application Priority Data Sept, 2t. 1966 Germany 5105968[52] US. Cl. 335/210; 250/396; 335/216 [5 1] Int. Cl. H01! 7/00 [58]FieldofSearch ..335/2l0,2ll,2l2,2l3, 335/214, 216; 250/495 [56}References Cited UNITED STATES PATENTS 3.008.044 ll/l96l Buchhold335/210 X 135L754 l H1967 Dietrich et al 335/210 X PrimaryE.\uminer-George Harris Armrney, Agent, or Firm-Curt M. Avery; Arthur E.Wilfond; Herbert L. Lerner [57] ABSTRACT A corpuscular ray device havinga magnetic lens assembly. Although the invention is particularlyapplica- 1 51 Aug. 19, 1975 ble to objective lens assemblies of electronmicroscopes which operate under vacuum, it is also applicable to lensassemblies for ion microscopes, electron diffraction devices. or othercorpuscular ray apparatus wherein the rays are situated in tubularenclosures. The magnetic lens assembly includes a pair of shieldingcylinders which are coaxial and spaced from each other, which are madeof a superconductive material. and which have their common axiscoinciding with a lens axis of the lens assembly, these shieldingcylinders being surrounded by a lens winding which may also be made of asuperconductive material and which serves to generate the magnetic fieldwhich is concentrated by the shielding cylinders in the region of thecorpuscular ray. These shielding cylinders have directed toward eachother a pair of end faces which are spaced from each other and definebetween themselves a lens gap which is devoid of any shieldingcomponents made of superconductive material. The aperture error constantof the lens assembly depends upon the maximum value of the fieldintensity in the lens gap and upon the field gradient along the lensaxis in the lens gap. The magnitude of the lens gap is chosen in such away that with a predetermined value of field intensity outwardly beyondthe lens gap and the shielding cylinders the maximum value of the fieldintensity in the lens gap and the field gradient in the lens gap alongthe lens axis provide for the lens assembly an aperture error constantwhich is less than a predetermined value of aperture error constant.

12 Claims, l2 Drawing Figures PATENTEDAUGISIQTS 3800808 Fig.1

III

9. CD b (11G) Fig.2

Fig.3

PATENTED AUG 1 9 I975 saw u 0F 4 Fig. 12

MAGNETIC LENS ASSEMBLIES FOR CORPUSCULAR RAY DEVICES WHICH OPERATE UNDERVACUUM Our invention relates to electromagnetic lens assemblies forcorpuscular ray devices, and in particular to objective lens assembliesfor electron microscopes.

Our invention in fact is a further development of the inventiondisclosed in copending US. Pat. application Ser. No. 389,089, filed Aug.12, 1964 now U.S. Pat No. 3,351,754.

Thus, it has already been proposed to provide a lens assembly whichincludes along the lens axis a pair of coaxial mutually spaced shieldingcylinders made of superconductive material and having their common axiscoinciding with the lens axis, these shielding cylinders being thermallyconnected with a cryogenic refrigerating medium. The shielding cylinderswhich are thus spaced from each other and range one after the otheralong the lens axis which coincides with the common axis of theshielding cylinders serve to concentrate in the region of thecorpuscular ray a magnetic field which is generated by a suitable numberof lens windings also made preferably of superconductive material andthrough which current flows.

In this proposed construction the two shielding cylinders, the end facesof which are directed toward each other and spaced from each other, formparts of a magnetic shielding structure which has generally spoken theshape of a hollow ring and contains in its hollow space the lenswindings. This magnetic shielding structure is madeof superconductivematerial and has its inner wall which is situated adjacent thecorpuscular beam divided in two shielding cylinders by a ringshapedopening lying in a plane perpendicular to the beam. This ring-shapedopening serves to accommodate an apertured disc which is superconductingand which has a substantially central aperture provided for thecorpuscular beam. Because of the presence of this ring-shaped opening inthe inner wall of the magnetic shielding structure said inner wallconsists of a pair of shielding cylinders which are spaced from eachother along the lens axis and which have end faces spaced from anddirected toward each other.

It is a primary object of our invention to improve the structure at theregion of the lens gap.

In particular, it is an object of our invention to provide aconstruction where the space between the end faces of the shieldingcylinders which define the lens gap is completely devoid of anycomponents made of superconductive material.

Also, it is an object of our invention to provide a construction whichwill reliably maintain the aperture error constant below a predeterminedvalue.

In addition, it is an object of our invention to provide shieldingcylinders with constructions at their mutually spaced ends which definethe lens gap which will greatly enhance the formation of a desiredmagnetic field.

Thus, our invention relates primarily to a particular configuration ofthe arrangement defined by the shielding cylinders which are made of thesuperconductive material. With the lens assembly of our invention thepair of end faces of the coaxial shielding cylinders which are directedtoward each other and which define between themselves the lens gap bylimiting the ends of the lens gap coact with a lens gap which iscompletely devoid of any shielding elements made of superconductivematerial. The magnitude of the lens gap is chosen in such a way that ata predetermined value of the field intensity outwardly beyond the lensgap and the shielding cylinders, the maximum value of the fieldintensity in the lens gap and the field gradient in the lens gap alongthe lens axis provide an aperture error constant of the lens assemblywhich is less than a predetermined value of this latter constant.

With the already proposed construction of the type referred to above,the shielding cylinders created by the ring-shaped opening in thering-shaped shielding structure serve only to prevent the formation ofthe field in the region of the corpuscular ray beyond a predeterminedlocation while a further superconducting element in the form of theabove-mentioned apertured disc serves to define this predeterminedlocation for the influencing of the corpuscular beam by the magneticlens field. In contrast, however, with our invention the construction ofthe shielding cylinders is such that the lens gap and the shape of themagnetic field within the lens gap are determined solely by the twoshielding cylinders. The functions performed by an apertured disc ofsuperconductive material of the type referred to above are instead takenover by the two shielding cylinders themselves, since it is theseshielding cylinders of our invention which determine the fieldrelationships at the corpuscular ray.

As a result, the construction of the lens assembly of our invention issimpler. At least by suitably dividing the lens windings it is possibleto have easy access to the lens gap. Moreover, where an apertured discof superconductive material is used, as in the already proposedconstruction, it is essential to refrigerate the disc with a cryogenicrefrigerating medium which is operatively connected to the disc from theoutside of the assembly, so that it is possible to collect the fieldlines in the region of the corpuscular ray, and of course all of theseinconveniences and complications are avoided with our construction.

Superconductive materials are used in the electromagnetic lensassemblies because the aperture error of the lens assembly whichinfluences the resolving power of the lens or corpuscular ray device ismaintained smaller the larger the magnetic field intensity in the regionof the corpuscular ray and the smaller the length of this field-filledregion. With conventional electromagnetic lenses which operate at normaltemperatures and which are provided with iron components for the returnflux path as well as pole shoes, these requirements, which actuallyconflict with each other in these conventional constructions, cannot befulfilled to a degree sufficient for high resolutions because of themagnetic properties of the iron. in the event that it is desired toachieve high magnetic field intensities in the region of the corpuscularray, with such conventional constructions, then in order to avoidsaturation of the iron large cross sections of the iron are essential,so that the axial length of that region in which the magnetic lens fieldacts on the corpuscular ray is undesirably enlarged.

In contrast, however, when superconducting materials are used it ispossible to achieve much higher magnetic field intensities withoutincreasing the axial length of the region where the magnet field acts onthe corpuscular ray to any appreciable degree.

The aperture error constant C, of an electromagnetic lens assemblydepends not only on the maximum value H, of the field intensity in thelens gap, which is to say the region where the field acts on thecorpuscular ray, but also on the field gradient along the lens axis inthe lens gap.

If an at least approximately bell-shaped field is taken as anillustrative example, then the field gradient for a given maximum valueI-I of field intensity in the gap has its magnitude determined by thewidth at one half the maximum intensity.

Our invention is illustrated by way of example in the accompanyingdrawings which form part of this application and in which:

FIG. 1 is a graphic illustration of the relationship between fieldintensity and width;

FIG. 2 is a schematic sectional elevation of one possible embodiment ofa structure of our invention;

FIG. 3 is a sectional elevation of another embodiment of a structure ofour invention;

FIGS. 4-9 respectively illustrate different configurations at the innerends of the shielding cylinders; and

FIG. is a longitudinal sectional elevation schematically illustratingyet another embodiment of a construction according to our invention.

FIGS. 11 and 12 show in diametrical section two further embodiments ofshielding cylinders according to the invention.

Thus, referring to FIG. 1, it will be seen that the width 2d at half themaximum field intensity is shown in the graph of FIG. 1. Thus, FIG. 1shows the horizontal coordinate z which corresponds to the lens axis andwhich has its starting point 0 forming the central plane of the lensgap, while the behavior of the field intensity I-I/I-I, in the gap,normalized on the maximum value H is indicated along the lens axis inFIG. 1. The width at half the maximum value of field intensity 2d is tobe understood, by definition, as the field width where H/H is equal to0.5.

It is apparent, therefore, that the width at half maximum fieldintensity depends upon the dimensions of the pair of shielding cylindersused according to our invention to form the lens gap, at the region ofthe end faces of these cylinders which are directed toward each other.This dependency applies not only with respect to the distance betweenthe pair of shielding cylinders, which is to say the length of the gap,but also with respect to the configuration of the shielding cylinders inthe region of their end faces which are directed toward each other.Since the shielding cylinders operate in such a way that over theirlength they prevent the approach of the electromagnetic lens field tothe corpuscular ray, while at the same time they are also to solve thetask of the above-mentioned apertured disc of superconductive material,that is to provide for entrance of the magnetic field to the largestpossible extent into the lens gap, a preferred construction of ourinvention provides for the end faces of the shielding cylindersconfigurations which conform to the course taken by the field lines. Inthis way high values of field intensity at critical locations of thematerial are avoided so as to also avoid the danger of flux jumping.Moreover, it is possible to provide for the shielding cylinders endfaces which are formed differently from each other so that the field isnot symmetrical in the gap.

For the same reasons a further construction of the shielding cylindersof our invention provides for the bore of the shielding cylindersthrough which the corpuscular ray travels a diameter which at least atthe lens gap region is as small as possible. By reducing thecrosssectional area of the bore of the shielding cylinders through whichthe corpuscular ray passes, at least in the region of the lens gap, thissize of the bore being reduced to the minimum value required for passageof the corpuscular ray, a very sharply defined guiding of the magneticfield in the lens gap and in particular in the region immediatelysurrounding the corpuscular ray with a definite field gradient isassured.

As has already been pointed out above, the aperture error constant C ofa lens assembly is very strongly influenced by the maximum value H, ofthe magnetic field intensity in the lens gap. It is therefore also afeature of our invention to situate in the lens gap additionalfield-generating lens windings so as to increase the maximum value H, ofthe field intensity in the lens gap, these additional windings thusaugmenting the lens windings which surround the shielding cylinderswhich define the lens gap. Such additional windings may also be providedto contribute to a predetermined field configuration, to reducehysteresis phenomena, or to make it possible to change the focal lengthof the lens by generating a counter-field.

As has already been pointed out above, the shielding cylinders preventformation of the magnetic field in the vicinity of the corpuscular ray,except for the region of the lens gap. For these reasons the length ofthe shielding cylinders is such that their ends which are directed awayfrom each other preferably extend into a region of negligible fieldintensity.

It may be desirable, however, in many cases, for the purpose of savingspace within the vacuum chamber of the corpuscular ray device, forexample, to connect to the pair of shielding cylinders further shieldingstructure made of superconductive material, so as to reduce theextension of the magnetic field generated by the lens windings.Preferably, these additional shielding structures take the form ofshielding structures which surround at all sides the surfaces of thelens windings which are arranged around the shielding cylinders exceptthe surfaces being opposite the shielding cylinders. This shieldingstructure is thus comparable to the ringshaped shielding of the alreadyproposed construction.

In order to avoid a considerable increase in the length of thefield-filled region of the corpuscular ray at the particular fieldintensity which is utilized, the shielding action of the pair ofshielding cylinders is required. Therefore, it is necessary for theseshielding cylinders to have a thickness selected according to thedesired field intensity.

Sintered Nb Sn has proved to be suitable as a material to be used forthe shielding cylinders and also for any further shielding structure.However, where the structure has relatively thick walls for theshielding cylinders which are made of this sintered material, in orderto avoid flux jumping, care must be taken to provide for the magneticfield intensity Fl, outwardly beyond the shielding cylinders and the gapa magnitude which increases at only a relatively slow rate after thestructure is put in operation. It is possible, however, to avoid thisfactor which reduces the speed of operation with a corpuscular raydevice which has this type of lens assembly, if porous sintered materialis used or if the shielding cylinders and also in given cases thefurther shielding structure take the form of preferably disc-shapedcarrier bodies made of heat-resistant material and coated with Nb Sn,this heat-resistant material being, for example, Nb, Pt orheat-resistant steel.

The superconductive materials can be used in connection with materialsof normal electrical and thermal conductivity by arranging thesuperconducting and normal conducting components one after the other inalternating sequence.

FIGS. 2-10 illustrate the more important features of differentembodiments of lens assmeblies according to our invention.

In the embodiments of our invention illustrated in FIGS. 2 and 3 thereis only a single field-generating lens winding 1. This lens winding 1surrounds the shielding cylinders 2a and 3a of FIG. 2 which have acommon axis coinciding with and extending along the coordinate 2 whichrepresents the lens axis. The shielding cylinders 2a and 3a are arrangedone after the other along the lens axis and are spaced from each otherso that they have end faces 4 and 5, respectively, which are spaced fromand directed toward each other and which form between themselves thelens gap S shown in FIG. 2. The pair of shielding cylinders 2a and 3a ofsuperconductive material thus prevent the formation of the magnetic lensfield generated by the winding 1 in the region of the lens axis 2, andthus in the region of the corpuscular ray, except for the space whichforms the gap S, and at this latter gap the shielding cylinders providefor the formation of a field of the greatest possible intensity.

It will be seen that the structure of FIG. 3 corresponds generally tothat of FIG. 2. However, in order to facilitate the formation of thefield in the region of the corpuscular ray within the lens gap S, FIG. 3shows a construction provided, as contrasted with that of FIG. 2, withshielding cylinders 2b and 3b which have end faces 6 and 7,respectively, directed toward and spaced from each other. These endfaces 6 and 7 respectively have configurations which conform to thecourse taken by the field lines within the lens gap.

Furthermore, the embodiments of FIGS. 2 and 3 differ from each other inthat the shielding cylinders 2b and 3b of FIG. 3 have a wall thicknesswhich is approximately four times as great as the wall thickness of theshielding cylinders 2a and 3a of FIG. 2. Experience has shown that withthis construction of FIG. 3 it is possible to have outside of theshielding cylinders a field intensity H, which is double that which ispossible with FIG. 2 without fearing any undesired increase in thelength of the field-filled region along the corpuscular ray.

Of course, insofar as the end faces of the shielding cylinders which aredirected away from each other may happen to still be situated in theregions of relatively high magnetic field intensity, it is possibleeither to increase the length of the shielding cylinders or to provideadditional shielding, and it is possible to increase the length so thatthe outer ends of the cylinders which are directed away from each otherare situated at regions of negligible field intensity. In some cases itis possible, particularly at very high field intensities, to resort toboth of these measures, namely increasing the length of the shieldingcylinders and providing additional shielding structure.

It will be noted that with the embodiment of FIG. 3, the end faces 6 and7 of the shielding cylinders 21: and 3b, respectively, have aconfiguration which conforms to the course taken by the field lines inthe lens gap S.

FIGS. 4-9 illustrate various possible constructions of this type whichhave proved in practice to be highly favorable.

Thus, FIG. 4 shows the shielding cylinder 2!; of FIG. 3 and inparticular the end face 6 thereof provided with the bevelledfrustoconical exterior surface portions 60, as is also illustrated inFIG. 3, so that in this way the end face 6 will conform to theconfiguration of the field lines. Of course, the other shieldingcylinder has its end face 7 constructed in the same way, and in FIGS.5-9 it is to be understood that the illustrated shielding cylinderscoact with identical shielding cylinders which have oppositely directedend faces.

Thus, in the case of FIG. 5 the shielding cylinder 2c is provided withan end face having a frustoconical surface 6b forming part ofa conewhose apex angle is substantially smaller than the apex angle of a conewhich includes the surface 6a of FIG. 4. Particularly in those caseswhere the field intensity H is relatively high outside of the gap, thisconstruction of FIG. 5 results in an increase in the maximum value H ofthe field intensity. However, this latter advantage is achieved onlywith a small increase in the length of the field-filled region along thecorpuscular ray.

In the embodiment of FIG. 6, the shielding cylinder 2d has a pair ofexterior frustoconical surface portions 60 and 6d which have differentangles so as to conform to an even greater degree to the course taken bythe field lines. In the embodiment of FIG. 7, the shielding cylinder 2ehas a rounded convex end face 6f surrounding the bore of the shieldingcylinder and situated at the end of a frustoconical surface 6e whichconforms generally to the surface 6b of FIG. 5.

On the other hand, in the embodiment of FIG. 8, the end face 63 of theshielding cylinder 2f is completely flat and situated in a plane whichis normal to the lens axis. However, in this embodiment the bore of theshielding cylinder terminates in the region of the lens gap in a reducedbore portion 1% which has an extremely small diameter just sufficientlygreat to permit the corpuscular ray to pass therethrough. Thisconstruction of FIG. 8 results in a large maximum value H, of the fieldintensity in the gap together with a small length of the field-filledregion along the corpuscular ray.

FIG. 9 shows a variation of the embodiment of FIG. 8 according to whichthe shielding cylinder 2g of FIG. 9 has the same bore portions 10a and101: as those shown in FIG. 8 while including the same exterior surfaceportions 62 and 6f as those shown in FIG. 7.

As is apparent from the features of FIGS. 4-9, the distance S betweenthe sheilding cylinders, the length thereof, and the configuration oftheir end faces which define and limit the lens gap must to a largeextent be empirically determined in accordance with the specialrelationships which are encountered in the particular corpuscular raydevice.

FIG. 10 shows an embodiment of our invention where the lens assembly isprovided with shielding cylinders 22 and 23 respectively correspondingto the above shielding cylinders, such as those shown in FIGS. 2 and 3.However, it will be noted that whereas in FIGS. 2 and 3 a single lenswinding 1 surrounds both shielding cylinders, in the embodiment of FIG.10 a pair of separate spaced lens windings 8 and 9 respectively surroundthe pair of shielding cylinders 22 and 23. These shielding cylinders 22and 23 of FIG. 10 are provided with additional superconductive shieldingstructures and 11.

In all embodiments the shielding cylinders are in thermal conductivitywith a cryogenic refrigerating medium, and in FIGS. 2, 3 and 10, thiscryogenic refrigerating means takes the form of containers c in which acryogenic medium such as liquid helium is located, for example.

The division of the lens winding means into a pair of individual lenswindings 8 and 9 as shown in FIG. 10 provide a gap S which is of easyaccessibility. It is thus possible to arrange in the gap S additionalwindings 12, as indicated in FIG. 10. However, it is also possible tosituated in this gap S a suitable specimen, a diaphragm, or a stigmatorelement, these elements being capable of introduction into the gap Sfrom the side. According to FIG. 11, the shielding cylinders may be madeby arranging disc-shaped components 30 of superconducting material andcomponents 31 of normal conductivity one after the other in alternatingsequence.

FIG. 12 shows a shielding cylinder avoiding flux jumping after thestructure is turned on. This cylinder comprises disc-shaped carrierbodies 40 made of heatresistive material and coated with layers 41 of NbSn.

Thus, our invention provides a further development or furthergeneralization of lens assemblies of the type already proposed byarranging and forming the shielding cylinders which are already presentin the already proposed assemblies in such a manner that they take overand perform the functions which are now performed by the additionalsuperconducting apertured disc of the already proposed assemblies.

We claim:

I. In a magnetic lens assembly for a corpuscular ray device which is tooperate under vacuum, such as an objective lens assembly of an electronmicroscope, a pair of coaxial shielding cylinders spaced from each otherand having a common axis coinciding with a lens axis of the assembly,said shielding cylinders being made of a superconductive material, acryogenic refrigerating means thermally connected with said cylinders,lens winding means surrounding said cylinders for generating a magneticfield, said cylinders concentrating said field in the region of acorpuscular ray traveling along said lens axis and said shieldingcylinders respectively terminating in a pair of end faces which aredirected toward each other and which define between themselves a lensgap which is devoid of any shielding components made of superconductivematerial, said gap having a magnitude which at a predetermined value offield intensity beyond said lens gap and said shielding cylindersprovides a maximum value of field intensity in the lens gap and a fieldgradient in the lens gap along said lens axis resulting in an apertureerror constant of the lens assembly which is less than a predeterminedvalue.

2. The combination of claim 1 and wherein said end faces of saidshielding cylinders which define said lens gap have a configurationwhich follows the course of field lines.

3. The combination of claim 1 and wherein said shielding cylinders arerespectively formed with axial bores passing therethrough and having atleast in the region of said lens gap an extremely small cross sectionalarea.

4. The combination of claim I and wherein additional field-generatinglens windings are situated at said lens gap.

5. The combination of claim 1 and wherein said shielding cylindersrespectively terminate distant from said end faces which defined saidlens gap in end faces which are directed away from each other and whichextend into a region of negligible field intensity.

6. The combination of claim I and wherein additional superconductiveshielding material is situated at the exterior of said lens windingmeans surrounding said shielding cylinders.

7. The combination of claim 1 and wherein said cylinders are ofdisc-shaped configuration.

8. The combination of claim 1 and wherein said shielding cylinders aremade of sintered Nb sn.

9. The combination of claim 1 and wherein said shielding cylinders aremade of disc-shaped carrier bodies of heat-resistant material coatedwith Nb Sn.

10. The combination of claim 1 and wherein components of superconductivematerial are connected with components which are of normal electricaland thermal conductivity.

11. The combination of claim 6 and wherein the additionalsuperconductive shielding material is surrounding at all sides the lenswindings except at the side being directed toward the shieldingcylinders.

12. The combination of claim 1 and wherein the lens windings consist ofsuperconductive material.

F l i

1. In a magnetic lens assembly for a corpuscular ray device which is tooperate under vacuum, such as an objective lens assembly of an electronmicroscope, a pair of coaxial shielding cylinders spaced from each otherand having a common axis coinciding with a lens axis of the assembly,said shielding cylinders being made of a superconductive material, acryogenic refrigerating means thermally connected with said cylinders,lens winding means surrounding said cylinders for generating a magneticfield, said cylinders concentrating said field in the region of acorpuscular ray traveling along said lens axis and said shieldingcylinders respectively terminating in a pair of end faces which aredirected toward each other and which define between themselves a lensgap which is devoid of any shielding components made of superconductivematerial, said gap having a magnitude which at a predetermined value offield intensity beyond said lens gap and said shielding cylindersprovides a maximum value of field intensity in the lens gap and a fieldgradient in the lens gap along said lens axis resulting in an apertureerror constant of the lens assembly which is less than a predeterminedvalue.
 2. The combination of claim 1 and wherein said end faces of saidshielding cylinders which define said lens gap have a configurationwhich follows the course of field lines.
 3. The combination of claim 1and wherein said shielding cylinders are respectively formed with axialbores passing therethrough and having at least in the region of saidlens gap an extremely small cross sectional area.
 4. The combination ofclaim 1 and wherein additional field-generating lens windings aresituated at said lens gap.
 5. The combination of claim 1 and whereinsaid shielding cylinders respectively terminate distant from said endfaces which defined said lens gap in end faces which are directed awayfrom each other and which extend into a region of negligible fieldintensity.
 6. The combination of claim 1 and wherein additionalsuperconductive shielding material is situated at the exterior of saidlens winding means surrounding said shielding cylinders.
 7. Thecombination of claim 1 and wherein said cylinders are of disc-shapedconfiguration.
 8. The combination of claim 1 and wherein said shieldingcylinders are made of sintered Nb3Sn.
 9. The combination of claim 1 andwherein said shielding cylinders are made of disc-shaped carrier bodiesof heat-resistant material coated with Nb3Sn.
 10. The combination ofclaim 1 and wherein components of superconductive material are connectedwith components which are of normal electrical and thermal conductivity.11. The combination of claim 6 and wherein the additionalsuperconductive shielding material is surrounding at all sides the lenswindings except at the side Being directed toward the shieldingcylinders.
 12. The combination of claim 1 and wherein the lens windingsconsist of superconductive material.